Continuous inkjet printers have been in commercial use for many years for labeling a wide range of products. According to the operating principle of the so-called continuous inkjet printer, ink that will be used in the printing is transported via, pumps out of a storage reservoir to a pressure chamber, under excess pressure, located in the actual print head, with said chamber having a nozzle on its side that faces the object to be imprinted. The nozzle has an opening diameter ranging from 30 μm to 200 μm, for example.
The ink jet is first emitted from the nozzle as a continuous ink jet; however this is not practical for labeling purposes, as the characters produced in this type of labeling are composed of individual dots or individual ink droplets.
To disperse the ink jet into individual, uniform ink droplets, a modulation element is attached to the pressure chamber, which generates pressure fluctuations in the emitted ink jet, causing it to break apart into individual, uniform ink droplets a short time after exiting the nozzle, at a specified distance.
Shortly before the ink droplets are separated from the emitted ink jet, each of the ink droplets is provided with an individual electrical charge, wherein the magnitude of the charge is based upon the desired impact position on the object to be labeled. To guarantee the electrical charge, the electrical conductivity of the ink is low. During the charging process, the ink droplet has not yet been separated from the ink jet emitted from the nozzle of the inkjet printer, and therefore, on the basis of electrostatic induction, free charge carriers in the ink are moved toward or away from the charging electrode, depending on the polarity and intensity of a charging voltage, wherein the ink chamber, and thus the ink reservoir, is held at ground potential, for example. In this case, the charging electrode has no mechanical contact with the ink jet.
If the ink droplet separates from the ink jet while it is within the field region of the charging electrode, the electrical charges that have migrated into the droplets as a result of electrostatic induction remain in the droplet volume, which has an external electric charge even after separation.
For example, if the charging electrode is positively charged, then when the ink jet enters the electrical field of the charging electrode, the negative free charge carriers in the ink migrate into the field, whereas the positively charged free charge carriers in the ink are forced out of the electrical field. Thus a charge separation takes place at the leading edge of the ink jet, immediately prior to separation of the droplet, and the charge imbalance produced in this manner is maintained in the separating droplet, and the droplet leaves the field region of the charging electrode negatively charged.
Because the ink droplet separates while the charging voltage is acting on the droplet, on the basis of structure and principle, a charge remains on the separated ink droplet, as described, the magnitude of which corresponds to the applied charging voltage, so that when the charging voltage changes, the charge magnitude on each droplet may also be changed.
Frequently, an electrode is also provided immediately downstream of the point of separation of the ink droplet, for detecting the charge actually provided in the droplet, whereby, for example, changes in the charging capacity of the ink and/or other external factors that influence the charge of droplets can be detected, and the charge levels of subsequent drops can be corrected during charging.
It is further known to allow the electrically charged ink droplets, on their initially linear trajectory, to pass into the electrostatic field of a plate capacitor located downstream, wherein they are deflected to a greater or lesser degree from their linear trajectory on the basis of their individual charges, and after leaving the electrostatic field, said droplets continue traveling at a specific angle in relation to their original trajectory, which is a function of their charge. With this system, it is possible to select different positions of impact on a surface to be labeled with individual ink droplets, wherein in this embodiment, this occurs in only one direction of deflection.
To remove individual droplets from the labeling image, or if printing will not be performed, the ink droplets are provided with a specific, fixed charge or remain uncharged, so that after they leave the electrostatic field of the plate capacitor, they strike a collecting tube, from which they are pumped back via a pump system into the ink tank. The ink not used for printing is thus circulated in a circuit, which forms the basis for the term continuous inkjet printer.
One advantage of this is that the usable inks can contain solvents, which evaporate within a very short time after printing on a print substrate, allowing processing of the imprinted objects to continue in a production line immediately after printing.
However, the rapid evaporation of the solvent can lead to problems, particularly inside the print head, if, for example, when the print head is shaken or malfunctions, ink reaches the electrodes or the interior of the print head in an uncontrolled manner, soiling these. Due to the short drying time of the inks, and the adhesivity thereof to the widest range of materials, which is particularly desirable for commercial applications, the interior, and particularly the electrodes located therein, can become so soiled over time that the electrical fields for charging, testing the charge of, and deflecting the ink drops can become altered, thereby impairing the print quality or even the functionality of the printer.
It is therefore necessary to clean the interior of the print head and particularly the electrodes located therein at regular intervals. A disadvantage of the prior art in this regard is that the interior of the print head forms a structured cavity traversed by the ink jet, in which electrodes, with their mounting supports and electrical connectors, are arranged raised and, together with the walls of the cavity, form a multitude of corners, edges and gaps, so that a simple and complete cleaning is complicated and time-consuming, and in some cases, only an incomplete cleaning is possible.
U.S. Pat. No. 4,743,922 A describes a system for a continuous inkjet printer comprising a print head having a housing that holds the components of the print head. The housing is made of at least two housing parts that can be separated and between which a cavity, that is traversed by a produced ink jet, is arranged.